The present invention relates to the field of spectroscopy with a plasma source. Known, but in no way limiting implementations of plasma sources within the field of this invention are chemical flames, DC arc and spark plasmas, RF induced plasmas as e.g. an inductively coupled plasma (ICP) or microwave plasmas, at reduced, at atmospheric or above atmospheric pressure. The invention described can be used for optical emission spectroscopy, where the optical emission of any of the plasma sources described above is used as radiation source or in conjunction with plasma-source mass spectrometry, where any of the plasma sources described above is used as an ion source. Further uses of the invention described, e.g. in the field of plasma generation for materials processing can be conceived and do not limit the scope of the patent desired.
The use of suitable plasmas as spectroscopic sources is probably as old as spectroscopy itself. Already in 1859, Robert Bunsen and Gustav Kirchhoff invented the “spectroscope” and with it the method of emission spectroscopy, using a chemical flame as source, which already in 1860 allowed them to discover the formerly unknown chemical elements cesium and rubidium in mineral water, by their characteristic emission spectra. At those times, the spectro-chemical source was observed directly in air, without any additional means of coupling the source to the spectrometer for throughput improvement.
In the 1930s, electrical arc and spark sources became popular for optical plasma emission spectroscopy, together with the use of chemical flames. Latest with the development of the inductively coupled plasma (ICP) as spectroscopic source, by Greenfield and Fassel in the 1960s, chemical flames increasingly were replaced by electrical discharges using RF excitation to sustain the plasma, typically in a suitable inert gas, e.g. Argon, into which the sample is introduced in the form of a fine aerosol, generated e.g. by pneumatic nebulization. The first ICP sources, typically operating with RF excitation at 27 or 40 MHz, employed a vertically oriented plasma with a “radial” or side-on pick-up of the plasma-generated radiation to be transferred into a suitable spectrometer.
In the mid-1970s, the first ICP spectroscopy instruments were introduced commercially, using a vertically oriented plasma. In the mid-1970s, an ICP source with horizontal plasma orientation and end-on pick-up of the ICP generated radiation was developed and subsequently commercialized.
Compared to the radial plasma radiation pick-up of the vertical ICP, the axially viewed, horizontally oriented ICP allowed for significantly improved signal-to-background ratios, mainly due to the larger observable size of the emission region.
However, since optical radiation with photon energies above ca. 6.2 eV is readily absorbed by air, an efficient use of the complete energy range of the radiation produced by the source is not possible without additional coupling means. Especially for an axially viewed plasma source, it can additionally be advantageous to modify characteristics of the spectro-chemical source to further improve the transfer of desired source radiation, to suppress background radiation, or to adapt electrical properties. Examples of source characteristics to be modified include the geometrical shape of the plasma, the removal of outer, colder source regions or the change of an electrical plasma potential.
In the case of plasma source mass spectrometry, typically also employing a horizontally oriented ICP, an efficient ion transfer mandates a direct coupling of the source to the spectrometer.
Accordingly, technical solutions for advantageously modifying and coupling a horizontally oriented plasma source to a spectrometer's optical system are described e.g. in U.S. Pat. No. 5,731,872 “Plasma Manipulator”, or specifically for a suitable modification of electrical plasma characteristics, in U.S. Pat. No. 5,841,531.
To fulfill the intended purpose, all technical solutions for modifying and coupling a horizontally oriented plasma source to a spectrometer's optical system require at least partial immersion of the device in the high temperature plasma. Typical spectro-chemical ICP sources, as an example, reach plasma temperatures of 6,000 -10,000K and are operated at RF powers between 1 and 2 kW.
A useable plasma modification and coupling device for a spectro-chemical plasma source thus requires a careful choice of material (e.g. resistance and electrical compatibility to the plasma environment), geometry (e.g. size and shape of the coupling device aperture, distance of the plasma coupling aperture from the spectrometer's optical system) and a suitable cooling (to avoid quick degradation of coupling device or optical components, or thermal instability of the spectrometer optics). So far, all known devices according to this purpose have been made from suitable conductive (metallic) material and have been liquid-cooled, specifically water-cooled. Not least due to the large heat capacity of typical, water-based cooling liquids, a very space-effective cooling of a plasma modification and coupling device for a spectro-chemical plasma source can be realized that further allows for device operation at or little above ambient temperature even at high plasma temperatures and powers, minimizing component wear and heat transfer to the spectrometer optics.
Increasingly, in the light of both environmental as well as economical concerns, liquid/water-cooling finds lessening acceptance. Additionally, liquid/water-cooling results in necessitating an additional resource for the operation of a plasma spectro-chemical device, unsatisfactory from a system complexity standpoint alone. Direct air-cooling of such device would be an ideal solution, however, due to the lower heat capacity of air, compared to water, achieving similar cooling power results in a larger heat exchanger and possibly higher operation temperatures for an air-cooled system.
Since the purpose of such plasma source modification and coupling device typically requires dimensioning of its size and positioning of optical elements in line with the source's or spectrometer's requirements, additional size or space constraints from a potentially larger air-air heat exchanger will typically result in less than optimal coupling. Additionally, higher operation temperatures, as often found in air-cooled systems can be detrimental for the performance and lifetime of delicate optical components, longer warm-up times (to reach a stable and higher operating temperature) negatively impact the long term stability of such device or an instrument incorporating such device. Both larger size and higher operation temperature are clearly undesirable for a spectro-chemical plasma modification and coupling device in the context of modern plasma spectro-chemical instrumentation.
Therefore, a need exists for a suitably air-cooled spectro-chemical plasma source modification and coupling device that replaces the undesirable liquid/water-cooling, remedying the presumed short-comings of air-cooled systems while preserving most or all advantageous aspects of existing implementations, most notably the optimum coupling geometry and the protection of the optical system from the high temperature plasma environment of the source.